1. Field of the Invention and Related Art Statement
The present invention relates to an electronic endoscope apparatus having an output circuit which delivers output signals of positive polarity from a solid-state imaging device.
In general, an endoscope apparatus has an elongated insert portion which is capable of being inserted into a body cavity for the purpose of observation of organs in the body cavity or for a medical treatment by means of a treating instrument inserted as required into an instrument channel of the insert portion. Such endoscope apparatus are finding spreading use in recent years.
Electronic endoscope apparatus also have been known in which a solid-state imaging device such as, for example, a CCD is mounted on the end of the insert portion. The electronic endoscope apparatus has a signal processing device and a monitor device. An example of such an electronic endoscope apparatus is disclosed in, for example, Japanese Patent Unexamined Publication No. 62-164383. An electronic endoscope apparatus shown in FIG. 1 also has been known. FIGS. 2 shows the construction of the known endoscope apparatus 1 shown in FIG. 1.
The endoscope apparatus 1 has an electronic endoscope (referred to as "electronic scope" hereinafter) 2, a light source unit 3 capable of applying illuminating light to the electronic scope 2, a control unit (signal processor) 4 for conducting signal processing on an imaging means of the electronic scope 2, and a monitor 5 for conducting a color display of the video signals derived from the control device 4.
The electronic scope 2 has an elongated insert portion 7 which is provided at its rear end with a manipulating portion 8 is a greater diameter. A universal code 9 is extended from this manipulating portion 8. A connector 11 secured to the end of the code 9 is connectable to the light source unit 3 and the control unit 4.
The insert portion 7 has a hard end section 12, a curved section 13 and a flexible section 14. The curved section 13 is capable of changing its curvature when manipulated through an angle knob 15 provided in a manipulating section 8.
The light source unit 3 has a concaved mirror which can collimate white light of a light source lamp and emit the collimated light. The collimated light is made to pass through a rotary color filter which is rotated by means of a motor 22 and is then converged through a condenser lens 24 so as to impinge upon an end surface of the light guide 25.
The color filter 23 has a rotary wheel in which are formed three sector-shaped openings which are covered by color-transmitting filters 26R, 26G and 26B of red, green and blue colors. As the rotary wheel rotates, these color transmission filters 26R, 26G and 26B are successively brought into the path of the light so that surface sequential illuminating lights of red, green and blue colors are supplied to the light guide 25.
The illuminating light is emitted from the end surface of a light guide 25 and is made to impinge upon a subject 28 through an illumination lens 27 so as to illuminate the subject 28. The image of the subject 28 is formed on a solid-state imaging device 31 disposed at the focal plane of an objective lens 29 which is attached to the end section 12 and which has a focal plane on the solid-state imaging device 31. The solid-state imaging device 31 is capable of conducting photo-electric conversion so as to convert the optical image into electrical signals.
The dive signal from a drive circuit 34 in the control unit 4 is supplied to the solid-state imaging device 31 so that the video signals are read from the solid-state imaging device 31 and are current-amplified by means of a buffer amplifier 35. The read signals ate then transmitted through a coaxial cable 36 and input to a buffer amplifier 37 in the control unit 4.
The electrical signals transmitted through the buffer amplifier 37 are sampled by a sample-hold circuit 38 so that the electrical signals obtained from the solid-state imaging device 31 are converted into video signals of the base band. Subsequently, the signals are made to pass through a gamma (.gamma.) correction circuit 39 and are converted into digital signals by means of the A/D converter 41. Then, the frame-sequential R, G and B signals are successively written in R, G and B frame memories 43R, 43G and 43B through a multiplexer 42. The signals written in the frame memories 43R, 43G and 43B are simultaneously read in accordance with a control signal output from a control circuit 44 and are converted into analog color signals R, G and B by D/A converters 45, whereby a color image of the subject formed on the solid-state imaging device 31 is displayed on the monitor 5.
The control circuit 44 is capable of producing control signals for controlling the operations of the driver 34. A/D converter 41, multiplexer 42 and the D/A converter 45.
As in the case of a conventional optical scope, the outside diameter of the end section 12 of the insert portion is preferably small while the length of the hard portion of preferably small, in order to widen the coverage of the endoscope. Thus, there is an increasing demand for reduced diameter and length of the hard end section of the electronic scope 2.
The length of the scope varies depending on the portion to which the endoscope is applied. It is necessary that the transmission be done in such a way as to minimize the loss of the high-frequency signal along the endoscope.
FIG. 3 illustrates a practical arrangement of the electronic scope. An emitter-follower-type or a source-follower-type buffer amplifier 33' is provided on the output of the CCD 31' as the solid-state imaging device 31, and transmission of the signals is conducted by an impedance matching with a transmission line 36 having a coaxial cable, through a matching resistor R. A line connected to a power supply V.sub.cc is grounded through a decoupling capacitor S.
A floating diffusion amplifier (FDA) method as shown in FIG. 4 is used for detecting signals on the CCD 31' presently used, i.e., for converting electrostatic charges into voltages. More specifically, in a system relying upon the FDA 47, the output of the CCD 31' is applied through a charge detection capacitor C1' to the gates of a MOSFET Q1' which constitutes an output amplifier. The source of the FET Q1' is connected to the output terminal of the FET Q1' and is grounded through a FET Q2' which constitutes a load. The gate of the FET Q1' is further connected to the output of a reset voltage V.sub.R through a resetting FET Q3'.
When a reset pulse .phi.R is applied to the gate of the resetting FET Q3'. the FET Q3' is turned on so that the capacitor C1' for holding the signal charged from the CCD 31' is reset by a resetting voltage V.sub.R. In this state, the level of the output end of the CCD is maximized. After the resetting, the voltage of the capacitor C' is changed from the resetting voltage V.sub.R in accordance with the quantity of the signal charges on the CCD 31'. For instance, when the level of the signal charges is close to the black level, the amount of change of the voltage from the resetting voltage V.sub.R becomes small. In this case, the level of the output from the output terminal approaches the maximum level. Conversely, when the level of the signal charges approaches the white level, the amount of change from the resetting voltage V.sub.R is increased so that the output level from the output terminal approaches the ground level.
In consequence, the signal output from the amplifier 47 shown in FIG. 4 has negative polarity as shown in FIG. 5. The negative polarity of the signals as shown in FIG. 5 causes a large loss of power in the FET Q1' because the voltage V.sub.SD between the source and drain of the FET Q1' is high and because the FET Q1' is so set as to perform an amplifying function so as to change the voltage V.sub.SD in response to a small change of voltage from the black level.
Therefore, a large loss of power is caused in the buffer amplifier 33' of FIG. 3, with the result that the temperature of the end section 12, in particular the CCD 31' or the solid-state imaging deice 31, is raised by the heat generated in the buffer amplifier 33'. This temperature rise causes an increase in the dark current, so that the image displayed on the monitor is seriously impaired due to roughening of the image.
FIG. 6 shows an arrangement for overcoming the above-described problem. In this arrangement, different electric currents are supplied to the CCD 31' and the buffer amplifier 33'. Namely, the power voltage V.sub.ccb of the emitter-follower type (or source-follower type) buffer amplifier 3' is set to be lower than the power voltage V.sub.cca of the CCF 31' , so as to reduce the voltage between the collector and the emitter of the emitter-follower buffer amplifier 33', whereby the power loss in the buffer amplifier 33' is reduced to suppress the temperature rise of the end section 12. This arrangement, however, requires that a de-coupling capacitor C2' is used in addition to the de-coupling capacitor C2'.
As described before, when a common power supply is used both for the solid-state imaging device and the buffer amplifier which provides signals of negative polarity, the dark current is increased due to heat generated in the buffer amplifier so that the image quality is impaired due to roughening of the display image.
On the other hand, when separate power supplies are used for the solid-state imaging device and the buffer amplifier, it is necessary to employ an additional cable for the supply, as well as a de-coupling capacitor C2'. This is quite inconvenient from the view point of reduction in the diameter and the length of the hard end section of the scope and, hence, an improvement is necessary in this respect.
Meanwhile, the specification of U.S. Pat. No. 4,814,648 discloses and arrangement in which, in order to remove the 1/f noise, the output signal from the CCD is supplied to an inverting amplifier circuit through a capacitor which functions as a high-pass filter for suppressing low-frequency components.
In this known arrangement, since the low-frequency components are cut, it is impossible to reproduce the D.C. level of the output signal from the CCD. In order to obviate this problem, this arrangement employs a clamp circuit (D.C. reproduction circuit) for reproducing the D.C. level. Thus, when a capacitor is used for the purpose of eliminating the 1/f noise, it is necessary to use a clamp circuit for reproducing the D.C. level. In addition, the circuit construction of the CCD output circuit is complicated. Furthermore, unfavorable effect is produced by the heat generated by the clamp circuit.